It may happen that a motorist, driving for example on the right lane of a highway, inadvertently encroach on the left lane crossing the marker line which separates the two lanes. This may create a dangerous situation, in particular when another vehicle is overtaking. Likewise, the motorist may also encroach on the hard shoulder for emergency use and cross the marker line that separates the right lane of the highway from said hard shoulder for emergency use. This can also create a dangerous situation, particularly when another vehicle is stationary on the hard shoulder for emergency use.
In order to warn the motorist of the dangerous situation awaiting him, it has already been proposed to replace the conventional marker lines, which are smooth with rough marker lines. When the wheels of a motor vehicle encroach on such rough marker lines, they produce a characteristic sound audible inside the passenger space of the vehicle, which tells the driver that he is drifting from his path. The noise stops as soon as the driver has corrected his path and returned to his lane.
The appearance of rough marker lines constituted real progress in road safety. Unfortunately, given the extent of road networks, not all roads can be fitted with such marker lines. Moreover, the noise produced by the wheels of the motor vehicle when they encroach on a rough marker line can be concealed by the noise of the engine or the sound of the car radio, so that the driver cannot be informed of the dangerous situation that he is in.
In order to overcome the drawbacks of passive security systems such as rough marker lines, car manufacturers have proposed active security systems on board the vehicles. Tests have thus been carried out with the aid of a camera installed under the vehicle and filming continuously the carriage way that passes in front of the vehicles's wheels. Such a system requires, however, a complex computer device to be able to analyse the images generated by the camera and to detect crossing of a separation line, which makes it particularly expensive and difficult to implement.
Another known active security system includes a multiplicity of infrared transmitters associated with photo-sensors and disposed under the front bumper face-bars of the motor vehicle. Each sensor of this system is formed of a master box comprising a transmitter and a receiver, and a slave box also formed of a transmitter and a receiver, which enables the electronics to be shared so as to be able to phase shift transmissions and prevent cross-talk of the sensors. In a preferred embodiment of this system, seven double sensors, i.e. seven pairs of master and slave boxes, are disposed at regular intervals from each other under the vehicle so as to cover the entire width of said vehicle.
The object of the system described hereinbefore is not to anticipate detection of a lateral drift of the vehicle, but to generate an alarm from the moment that one of the wheels of said vehicle crosses a horizontal lane demarcation marker. Thus, the transmitters each emit an infrared light beam towards the carriageway. These infrared light beams are then reflected on the ground and directed towards the photo-sensors which, as a function of the quality of the reflected light, are capable of determining whether the light was reflected on the carriageway surface (for example concrete) or on a white strip of horizontal marking. As a function of the information collected by this system and the state of the vehicle (speed, whether there is prior indication of a manoeuvre by switching on turn signals or blinkers), said system actuates one or other of two vibrators located under the driver's seat to indicate to him that he is crossing a horizontal lane demarcation marker to the right, or respectively to the left.
The path monitoring system described hereinbefore has the advantage of quickly informing the driver of significant drifts in the path of his vehicle with respect to his lane on the carriageway, the causes of such drifts being varied: drowsiness, reduced vigilance, distraction. Such a system is less expensive than a system using a path-follower camera, but remains nonetheless relatively complex to implement. In fact, this system includes no less than fourteen master and slave boxes, which makes cabling of the system particularly complex and considerably increases the assembly time and number of parts necessary for assembly under the vehicle's bumper face-bar. This multiplication of the number of elementary sensors also raises the problem of the long-term reliability of such a system. Moreover, a double sensor formed of a master box and a slave box forms an indissociable unit insofar as the operating features of the master box are calibrated with respect to those of the slave box. Thus, in the event of a failure in a double sensor following a breakdown or damage to the front bumper face-bar, the master and slave boxes have to be replaced simultaneously. Consequently, the master and slave boxes have to be stored in pairs, which raises inevitable problems of stock management.
It is an object of the present invention to overcome the aforementioned problems in addition to others still, by providing a system for monitoring the path of a motor vehicle, which enables, in particular, the number of parts used to be limited.